1. Field of the Invention
The present invention relates generally to a Code Division Multiple Access (CDMA) communication system, and in particular, to a system and method for retransmitting uplink data according to a channel environment of a user equipment (UE).
2. Description of the Related Art
With the development of communication technology, asynchronous CDMA communication systems are evolving into high-speed packet data communication systems such as a High Speed Downlink Packet Access (HSDPA) communication system. The HSDPA communication system represents a communication system supporting a data transmission scheme including control channels related to a high speed-downlink shared channel (HS-DSCH) for supporting high speed downlink packet transmission in a Universal Mobile Telecommunications System (UMTS) communication system chiefly developed in Europe. In order to support the HSDPA scheme, an Adaptive Modulation and Coding (AMC) scheme, a Hybrid Automatic Retransmission Request (HARQ) scheme, and a Fast Cell Section (FCS) scheme have been proposed. A structure of a Wideband Code Division Multiple Access (WCDMA) communication system, i.e., a typical UMTS communication system, will now be descried with reference to FIG. 1.
FIG. 1 is a diagram schematically illustrating a structure of a conventional WCDMA communication system. The WCDMA communication system comprises a core network (CN) 100, a plurality of radio network subsystems (RNSs) 110 and 120, and a user equipment (UE) 130. Each of the RNSs 110 and 120 comprise a radio network controller (RNC) and a plurality of Node Bs (also called “cells” in the following description). More specifically, the RNS 110 comprises an RNC 111 and a plurality of Node Bs 113 and 115, and the RNS 120 comprises an RNC 112 and a plurality of Node Bs 114 and 116. The RNCs are classified as either a Serving RNC (SRNC), a Drift RNC (DRNC), or a Controlling RNC (CRNC), according to their functions. The SRNC and the DRNC are classified according to their functions for each UE. An RNC that manages information on a UE and controls data exchange with a core network is an SRNC, and when data of a UE is transmitted to the SRNC, not directly but via a specific RNC, the specific RNC is called a DRNC of the UE.
The CRNC represents an RNC controlling each of Node Bs. For example, in FIG. 1, if the RNC 111 manages information on the UE 130, it serves as an SRNC of the UE 130, and if data of the UE 130 is transmitted via the RNC 112, due to movement of the UE 130, the RNC 112 becomes a DRNC of the UE 130. The RNC 111 controlling the Node B 113 becomes a CRNC of the Node B 113.
With reference to FIG. 1, a description will now be made of the HARQ scheme, particularly an n-channel Stop And Wait Hybrid Automatic Retransmission Request (n-channel SAW HARQ) scheme. The n-channel SAW HARQ scheme is a newly introduced scheme, which utilizes a soft combining scheme and an HARQ scheme to increase efficiency of a common Stop And Wait Automatic Retransmission Request (SAW ARQ) scheme. The soft combining scheme and the HARQ scheme will now be described below.
1. Soft Combining
In the soft combining scheme, a receiver temporarily stores defective data in a soft buffer and then combines it with retransmitted data of the corresponding data to reduce an error rate. The soft combing scheme is classified into a Chase Combining (CC) scheme and an Incremental Redundancy (IR) scheme.
In the CC scheme, a transmitter transmits data using the same format for both initial transmission and retransmission. If m symbols were transmitted as one coded block for initial transmission, the same m symbols are transmitted as one coded block even for retransmission. The “coded block” refers to user data that is transmitted for one transmit time interval (TTI). That is, in the CC scheme, the same coding rate is also applied to initial transmission and retransmission. Then a receiver combines an initially transmitted coded block with a retransmitted coded block, and performs a cyclic redundancy check (CRC) operation using the combined coded block to detect occurrence of any possible error.
In the IR scheme, a transmitter uses different formats for initial transmission and retransmission. If n-bit user data was generated into m symbols through channel coding, the transmitter transmits only some of the m symbols at initial transmission, and sequentially transmits the remaining symbols at retransmission. That is, a coding rate for initial transmission is different from a coding rate for retransmission. A receiver then forms a coded block with a high coding rate by adding the retransmitted symbols to a rear part of the initially transmitted coded block, and performs error correction on the combined coded block. In the IR scheme, the initial transmission and each of the retransmissions are identified by version numbers. For example, initial transmission is assigned a version number #1, first retransmission is assigned a version number #2, and second retransmission is assigned a version number #3, etc., and the receiver can correctly combine an initially transmitted coded block with a retransmitted coded block using the version information.
In addition, the IR scheme is classified into a partial IR scheme and a full IR scheme. In the partial IR scheme, partial information on a format used for initial transmission is used in the same way during retransmission, and in the full IR scheme, totally different formats are used for initial transmission and retransmission. When the full IR scheme is used, it is possible to acquire a maximum gain with redundancy information, but in a certain full IR scheme, it is impossible to decode received data with only the retransmitted data. Such a characteristic is called a “non-self-decodable” characteristic. When channel coding is performed using a turbo encoder, systematic bits are not punctured during initial transmission. Therefore, if retransmission is performed using the full IR scheme, the systematic bits are not retransmitted. In this case, if the number of bits of retransmission data comprised of only parity bits is not relatively larger than a size (the number) of information bits before channel coding, the retransmission data is not self-decodable. Therefore, when the non-self-decodable retransmission data is transmitted, the receiver should always soft-combine the initially transmitted data with the retransmitted data, for normal data reception.
2. HARQ
In the common SAW ARQ scheme, a Node B does not transmit a next packet until acknowledgement (ACK) information for a previously transmitted packet is received. Because a Node B transmits a next packet only after ACK information for a previous packet is received, the Node B should occasionally wait for the ACK information even though it can currently retransmit a packet. However, in the n-channel SAW HARQ scheme, a Node B can continuously transmit a plurality of packets even before receiving ACK information for a previous packet is received, thereby increasing utilization efficiency of a radio link. That is, in the n-channel SAW HARQ scheme, n logical channels are set up between a UE and a Node B, and are identified by unique time slots or channel numbers, so the UE can determine to which channel a packet received at a particular time belongs. Therefore, the UE can take necessary measures to rearrange packets in a correct order and soft-combine the corresponding packets.
An operation of the n-channel SAW HARQ scheme will now be described in detail with reference to FIG. 1. First, it will be assumed that an n-channel SAW HARQ scheme, particularly a 4-channel SAW HARQ scheme, is performed between a UE 130 and a particular Node B, e.g., a Node B 114, and the 4 channels are uniquely assigned logical identifiers #1 to #4, respectively. In addition, the UE 130 and the Node B 114 include HARQ processors corresponding to the respective channels. The Node B 114 assigns a channel identifier #1 to an initial transmission coded block, before transmitting the initial transmission coded block to the UE 130. The channel identifier can be either uniquely assigned or implied as a unique time slot. If an error has occurred in a coded block transmitted with a channel identifier #1, the UE 130 delivers the defective coded block to an HARQ processor #1 corresponding to the channel identifier #1, and transmits negative ACK (NACK) information to the Node B 114. Then the Node B 114 can transmit a next coded block over a channel #2 regardless of whether the ACK information for a coded block of a channel #1 is received.
If an error has occurred even in the next coded block, the Node B 114 sends even the next coded block to a corresponding HARQ processor. If NACK information for the coded block of the channel #1 is received from the UE 130, the Node B 114 retransmits a corresponding coded block over the channel #1. The UE 130 then senses that the retransmitted coded block is retransmitted data of a coded block previously transmitted over the channel #1 through a channel identifier of the retransmitted coded block, and sends the retransmitted coded block to an HARQ processor #1. Upon receiving the retransmitted coded block, the HARQ processor #1 soft-combines the retransmitted coded block with the initially transmitted coded block already stored therein.
As described above, in the n-channel SAW HARQ scheme, channel identifiers are matched to HARQ processors on a one-to-one basis, so that a Node B can appropriately perform initial transmission and retransmission without delaying user data until ACK information is received.
In order to efficiently use the HARQ scheme in the manner described above, the HSDPA communication system divides the HARQ protocol stack into two layers. That is, in the HSDPA communication system, a soft buffer necessary for soft-combining data and an error correction function are located in a physical layer, and a function of determining ACK/NACK information and determining whether to perform soft combining by receiving ACK/NACK information is located in a media access control (MAC) layer.
A UMTS terrestrial radio access network (UTRAN) comprises a Node B and an RNC as illustrated in FIG. 1. In this structure, a physical layer is located in a Node B, and unlike the conventional MAC layer, a MAC layer of the HSDPA communication system, i.e., a MAC-hs (MAC-high speed) layer, is located in the Node B. The MAC-hs layer is a layer newly proposed for the HSDPA communication system, and controls an ACK/NACK information processing function for supporting the HARQ scheme. The HSDPA communication system locates the ACK/NACK information processing function in a Node B so as to perform fast HARQ processing.
Alternatively, a control operation can be performed so that the ACK/NACK information processing function is located in an RNC, and in this case, ACK/NACK information is delivered to the RNC via a Node B, and the RNC determines whether to perform retransmission depending on the ACK/NACK information provided via the Node B, and sends the determined result back to the Node B. The Node B then actually determines whether to perform data retransmission based on the determined result provided from the RNC. In this case, a delay time required for HARQ signaling (or signaling for performing the HARQ scheme) between a Node B and an RNC occurs. The delay time for HARQ signaling between a Node B and an RNC accounts for one frame, or 2 ms, which is a relatively long delay time. In order to minimize the delay time for HARQ signaling, the HSDPA communication system performs a control operation so that a Node B performs an ACK/NACK information processing function.
Currently, active research is being carried out on an uplink communication system for improving uplink communication efficiency together with the HSDPA communication system. That is, active research is being conducted on an uplink communication system that enables uplink data transmission using an enhanced uplink dedicated channel (EUDCH), which is an uplink data transmission channel. The uplink communication system using EUDCH can apply the data transmission schemes used in the HSDPA communication system. That is, the uplink communication system using EUDCH can employ the AMC scheme and the HARQ scheme adopted in the HSDPA communication system, and can use a relatively shorter TTI than that of the HSDPA communication system. The TTI, as described above, is a unit time interval for which one coded block is transmitted, and scheduling for downlink channels is performed by a Node B to prevent a scheduling delay.
As described above, the uplink communication system using EUDCH transmits data in an uplink direction, and must support the HARQ scheme for the data transmitted in the uplink direction as described in connection with the HSDPA communication system. However, detailed proposals have not been made for the uplink communication system using EUDCH, and detailed proposals for supporting the HARQ scheme also have not been made.